The impulse for the movement of electrons from NADH to O2 in the respiratory chain on the inner mitochondrial membrane is similar to the voltage source in a galvanic cell. Protons are pumped at three sites, the NADH-Q reductase complex, cytochrome reductase, and cytochrome oxidase. The movement of electrons from FADH2 to O2 moves only two protons because FADH2 releases its electrons directly to Q (ubiquinone), bypassing NADH-Q reductase. The resistive load or work coupled to this expenditure of free energy (current flowing through a positive cell potential) is the pumping of protons from the matrix into the intermembrane space. The respiratory chain pushes the protons uphill against a pH gradient of 1.4 and against a membrane potential of .14 V (electric field across the inner mitochondrial membrane). The movement of protons from the matrix to the intermembrane space stores free energy across the inner mitochondrial membrane. The free energy change associated with each electron movement (1) NADH to NADH-Q reductase (2) cytochromes b to c1 in cytochrome reductase (3) cytochrome a to O2 is sufficient to move a proton across the membrane against a free energy gradient. The energy stored in each such proton movement is sufficient to drive ATP synthesis with oppositely directed proton flow.
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